Mixing devices

ABSTRACT

A mixing device is proposed comprising a chamber defined by two circular cylindrical lobes in part overlapping parallel relation. Each lobe houses a similar rotor rotatable about the lobe axis, the rotors being maintained in a predetermined intermeshing contra-rotating relation. Each rotor has like symmetrical section along its length defined by a pair of radial arms. Each arm has concave sides, with adjacent ones of these sides merging into convex axial portions, and radially outer peripheries which pass closely adjacent the lobes surfaces. This geometry defines leading and trailing peripheral tips whereby each leading tip wipes its lobe and a trailing tip side of the other rotor, each trailing tip wipes the leading tip side of the other rotor, and each arm periphery rolls around an axial portion of the other rotor. Material to be mixed is charged and discharged axially for batch or continuous operation, and the rotors can provide a screw action for the latter operation. Similar arrangements involving more lobes in the chamber, and/or more arms for each rotor, can be evolved.

ilnited States Patent [191 Cheng et al.

[54] MIXING DEVICES [75] Inventors: David Chung-Hsun Cheng, Letchworth, England; John Brian Davis, Dumfries, Scotland [73] Assignee: National Research Development Corporation, London, England [22] Filed: Feb. 2, 1971 [21] Appl. No.: 112,010

[30] Foreign Application Priority Data FOREIGN PATENTS OR APPLICATIONS 530,321 7/1955 Italy ..259/l04 173,457 l/l922 GreatBritain ..259/l04 1 May 22, 1973 Primary Examiner-Robert W. Jenkins Assistant Examiner-Philip R. Coe Attorney-Cushman, Darby & Cushman [57] ABSTRACT A mixing device is proposed comprising a chamber defined by two circular cylindrical lobes in part overlapping parallel relation. Each lobe houses a similar rotor rotatable about the lobe axis, the rotors being maintained in a predetermined intermeshing contrarotating relation. Each rotor has like symmetrical section along its length defined by a pair of radial arms. Each arm has concave sides, with adjacent ones of these sides merging into convex axial portions, and radially outer peripheries which pass closely adjacent the lobes surfaces. This geometry defines leading and trailing peripheral tips whereby each leading tip wipes its lobe and a trailing tip side of the other rotor, each trailing tip wipes the leading tip side of the other rotor, and each arm periphery rolls around an axial portion of the other rotor. Material to be mixed is charged and discharged axially for batch or continuous operation, and the rotors can provide a screw action for the latter operation. Similar arrangements involving more lobes in the chamber, and/or more arms for each rotor, can be evolved.

10 Claims, 11 Drawing Figures MIXING DEVICES This invention relates to mixing devices and more particularly, but not exclusively, such devices for homogenizing and blending paste-like materials, such as butter.

In homogenizing and blending viscous or paste-like materials such as butter, it is necessary to design a mixing device in which the work in shear per unit volume is high but not high enough to give rise to excessive heating, and in which the desired degree of mixing is accomplished in a reasonable time. Presently available mixing devices are not completely satisfactory in that they can take up to twenty minutes per load to achieve an acceptable degree of mixing.

An object of the present invention is to provide an improved mixing device which may better satisfy the above requirements or which can be used with advantage for other mixing operations. To this end there is provided a mixing device comprising: a hollow housing defining a cylindrical chamber having symmetrical cross section formed by a plurality of like lobes with circular arcuate peripheries, the circular central axes of adjacent lobes being mutually displaced by a distance less than the sum of the radii thereof, and said housing having port means in at least one end thereof for passage of material to and from said chamber; a plurality of similar rotors individually mounted in respective ones of said lobes for rotation about said central axes, each rotor having like cross-sectional form along its axis defined by a plurality of like radially extending arms, each of said arms being defined, in turn, by generally concave radially extending sides and a radially outer periphery which passes closely adjacent the respective lobe periphery during rotation, and said concave sides of adjacent ones of said arms in each rotor merging in a convex axial portion; and means coupling said rotors for rotation in predetermined intermeshed relation with adjacent rotors rotating at the same rate but in mutually opposite senses.

In practice the sectional geometry of the rotors will be such that the sides of each arm meet their common periphery in generally acute tips to provide wiping or scavenging edges along the lengths of the rotors. These edges can be distinguished as between leading and trailing edges relative to the direction of rotation, and the leading edges serve to wipe the chamber lobes and collect material to be mixed. The sectional geometry is further such that this collected material is subjected to a compacting action. This action involves wiping of the arm side on which material has been collected by the trailing edge of an arm of an adjacent rotor, while the periphery of the latter arm effectively rolls and slides over the convex axial portion with which the former arm merges. This will be better understood from the further description hereinafter, but it remains to note here that the compaction squeezes the collected material through small inter-rotor or rotor/chamber clearances giving a very efficient mixing action.

The rotors can be cylindrical, or successive portions of each rotor along its axis can be angularly displaced, about such axis in a progressive manner, or the rotors can be smoothly twisted about their axes, such as in helical manner.

The mixer can be used for batch operation or continuous operation. In this last case there will be port means at both ends of the housing and the use of screwlike forms of rotor may be preferred to induce flow of material to be mixed through the mixing chamber.

Also, the rotors may be of hollowed form for the passage of heated or coolant fluid therethrough in order to afford heat exchange between the relevant fluid and material being mixed.

For clearer understanding of the present invention, the same will now be more fully described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 diagrammatically illustrates in side view one embodiment of a mixer according to the invention,

FIG. 2 shows an idealized sectional form for the rotors of the mixer of FIG. 1,

FIGS. 3a to 3e illustrate successive relative dispositions for a pair of rotors of the form of FIG. 2,

FIG. 4 illustrates a part view from one end of the embodiment of FIG. 1 and shows rotors of modified sectional form relative to that of FIG. 2, and

FIGS. 5, 6 and 7 diagrammatically illustrate rotors subject to yet further modification.

In the mixer of FIG. 1, the housing 10 defines a cylindrical chamber 1 1 formed by two identical part circular cylindrical lobes Ila and 11b which provide symmetrical 8-shaped cross-sectional form. For convenience of manufacture, the housing is made up of a body part 10a which serves to define the curved surfaces of the lobes, and end plates 10b and which are bolted or otherwise suitably connected to the body part 10a in sealing relation.

The mixer is provided with two identical rotors 12a and 12b respectively mounted for rotation about the central axes of the lobes 11a and 11b. Each of these rotors has two diametrally opposed arms having general geometrical form as described above and discussed in more detail hereinafter. The rotors have stub shaft projections 13 at their ends, which shafts are mounted for rotation in the end plates of the housing and pass in effective sealing relation through at least one of these plates.

Outside the housing, the rotor shafts are coupled for rotation in predetermined phase relation. This coupling is denoted by intermeshed gear wheels 14 and 15 respectively fixed on the upper shafts of rotors 12a and 12b, although any other suitable drive coupling can be employed. In any event, one of the shafts is further coupled with a drive motor 16 whereby the rotors are driven in mutually opposite senses by way of their shafts and gear coupling.

Lastly in FIG. 1, the end plates of the housing are provided with ports with which conduits 8 communicate for passage of material to and from the mixing chamber. The provision of ports and conduits in association with both end plates assumes passage of material through the chamber for continuous operation of the mixer. Clearly, it may be sufficient to port only one of the end plates for the purposes of charging and discharging in batch operation.

Turning to consideration of the more particular geometry of the rotors in FIG. 1, this is shown in what can be regarded as an idealized sectional form in FIG. 2. Referring then to FIG. 2: the relevant form of rotor has a pair of diametrally opposed, radially projecting arms merge to form a convex shaping 23 around the axial portion of the rotor..

More specifically in this idealized geometry:

i. the peripheries 22 are defined as circular arcs of radius R, equal to the lobe radius, and subtended angles of 90 at the center of the section, namely the rotor axis of rotation,

ii. the convex shapings are defined by circular arcs of radius R where R e R, and e I, and these arcs also each subtend angles of 90 at the section center,

iii. the remaining portions of the sides 21 are of curved form defined by the equations:

andy=rsin and 0 arc cos (1 +e/2) In order to balance conflicting requirements for large material handling capacity and mechanical strength, a value for e of approximately 0.3 is preferred.

FIGS. 3a to 32 show successive positions of the rotors having sectional geometry as in FIG. 2, and in a mixer as described with reference to FIG. 1. The rotors are designated as 12a and 12b, and are arranged for rotation in mutually opposite senses with their axes parallel and spaced by a distance (R, R the rotors being housed in a vessel with an inner surface defined by the peripheral paths of rotation 9 of the rotors.

Consideration of the successive rotor dispositions at (a) to (e) in FIG. 2 indicates the advantageous features of operation in that the leading tips of the rotor arms sweep the inner surface of the vessel and collect material to be mixed, which collected material is positively compressed between the rotor concave side surface adjacent the relevant tip and the convex peripheral surface of the other rotor which passes thereacross. Also, the trailing concave side surface of each rotor arm is swept by the leading tip of an arm.

Now embodiment of the geometry of FIG. 2 in the mixer of FIG. 1 would lead to wear over the whole of the rotor arm surfaces, with more severe wear at the tips due to their longer period of engagement with other surfaces during a cycle of rotation. Accordingly, during practical development of the invention, a first prototype model was made generally as illustrated by FIGS. 1 and 2, but with the rotor arm radius reduced slightly, as indicated in broken line in FIG. 2, to give a small clearance between the two rotors and also between the rotors and vessel. It was also considered that such a clearance would allow shearing of the material being mixed to take place between the rotors and the vessel.

However, the result of the sweeping action of the tips is impaired by the provision of a clearance. Also, the desired shearing action is small in the case of .pastes since they act more like a plastic than a viscous material, thus tending to stick to the vessel without suffering appreciable deformation.

In these circumstances, the prototype form was modified as shown in FIG. 4 by filling out the initial clearance with strips as denoted at 17 at the leading tips of the rotor arms. This modification gives rise to the ini-' tially contemplated advantageous sweeping action, while reducing the areas of wear and facilitating reestablishment of the regions of greater wear, namely, the arm tips.

In practice, the tips 17 can be made of metal, or plastics material, or a combination of both. These strips can be permanently secured to the rotors, this being probably best suited to use of metal tips, when welding and like techniques or even integral fabrication with the rotor will be appropriate. Alternatively, the tips can be releasably secured as by bolts or equivalent means, this being probably best suited to use of plastics material strips in association with metal backings.

FIG. 4 also further particularizes the disposition of the ports in the end plates of the mixer housing. The inlet port is indicated in broken line at 18 and is of slot form with its axis parallel to and laterally displaced from the common diameter of the two lobes to bridge the lobe spaces. For outlet purposes it is preferred to employ twin outlet ports as indicated at 19 in a region of high pressure during operation of the mixer. This region lies between the rotor axes but off-set from the common diameter towards the direction from which the rotor leading edges approach.

A further consideration in rotor geometry is that of its form in the axial direction. .It has been indicated above that this can be cylindrical and such a rotor is indicated in FIG. 5. The rotor of FIG. 5 is further particularized by being hollow, with the rotor hollow communicating with hollow stub shafts at its ends. These shafts will, in turn, communicate with conduits for passage of heated or coolant fluid through the rotor so that the rotor affords a heat exchange function.

FIG. 6 shows another form of rotor modified from that of FIG. 5 in that successive portions along the axial direction are relatively angularly displaced about the axis in a progressive manner.

FIG. 7 shows a yet further rotor modification in which the rotor is smoothly twisted to spiral or screw form along its axis. This twisting can be of a uniform nature to provide a helical screw form.

The rotor forms of FIGS. 6 and 7 are useful for continuous through-flow mixing operations by assisting .axial displacement of mixed material. However, this through motion is detrimental to an ideal mixing action and the choice of relative angular displacement or spiral pitch should be chosen to compromise between these requirements. Experiment indicates that a helical spiral pitch of one turn in 211' R tan 60 axial length is useful in the present context.

As a further indication of experimental results in developmentof the invention, it is useful to note data obtained from a pilot model of an embodiment as described with reference to FIGS. 1 and 4. This model had cylindrical rotors with diameters of 7.25 inches, heights of 4 inches, and central convex portions of 2.50 inches diameter. The axes of these rotors were spaced apart by 5 inches and the clearance between the rotor arms and the vessel, apart from at the filled-out tips, was 0.125 inches. Batch operation of this model with rotor rotation at 7 rpm. was sufficient to mix butter, at 0C, half filling the mixer, and half the butter being colored with an oil-soluble blue dye, within 2 3 minutes to produce a mixture of uniform color to the eye.

The power requirement of this pilot operation was 0.3 hp. at the end of mixing, and the weight of the mixed butter was 5 lbs. In a conventional mixer, with Z-formed blading, the power requirement is about 40 50 hp. when dealing with a batch of X 56 840 lbs. If one scales up the pilot operation on the basis of equal power requirement per pound of butter, the present mixer would require about 50 hp. at the start falling to about 25 h.p. at the end of mixing. Such a result would show no significant saving in power, but would afford a significantly shorter mixing time compared to the conventional operation.

While the invention has been described with particular reference to mixing butter it is not intended solely for such application. For example, another area of application is that of screw mixer/conveyor arrangements such as used for the extrusion of plastics materials. The invention can find application in the later stages of such arrangements where the plastics material is in a plastic or molten state and will assist in affording uniformity of extruded material.

Also, while the more particular description of drawings involves two-lobe chambers and double-armed-rotors, the principles of the invention can clearly be extended to more complex configurations involving more than two lobes and/or more than two arms on each rotor. Such configurations will, of course, be subject to affording contra-rotation between circumferentially adjacent rotors.

We claim:

1. A mixing device comprising: a hollow housing defining a cylindrical chamber having symmetrical cross section formed by a plurality of like lobes with circular arcuate peripheries, the circular central axes of adjacent lobes being mutually displaced by a distance less than the sum of the radii thereof, and said housing having port means in at least one end thereof for passage of material to and from said chamber; a plurality of similar rotors individually mounted in respective ones of said lobes for rotation about said central axes, each rotor having like transverse radial cross-sectional form at substantially all points along its axis defined by a plurality of like radially extending arms, each of said arms being defined, in turn, by generally concave radially extending sides and a generally convex radially outer periphery which passes closely adjacent the respective lobe periphery during rotation, and said concave sides of adjacent ones of said arms in each rotor merging in a convex axial portion; and means coupling said rotors for rotation in predetermined intermeshed relation with adjacent rotors rotating at the same rate but in mutually opposite senses; wherein the cross-sectional form of said rotors, and said intermeshing relation, are

such that: in each arm the junction of the periphery and sides define a leading tip and a trailing tip relative to the relevant direction of rotation; each leading tip wipes the periphery of its lobe and the trailing tip side of an adjacent rotor arm; and each trailing tip of one arm wipes the leading tip side of an adjacent rotor arm while the periphery of said one arm effectively rolls on and slides over the convex axial portion with which said leading tip side merges.

2. A device according to claim 1 wherein said arm peripheries and said convex axial portions at least approximate to circular arcuate sectional form.

3. A device according to claim 2 wherein the ratio between the radii of curvatures of said arm peripheries and convex portions is 1 0.3.

4. A device according to claim 1 wherein each rotor comprises a pair of arms, the 2 of said arms and said convex portions each subtend an angle of substantially at the rotor axis, and each arm side at least approximates in sectional form to a curve defined by the equations:

R is the rotor arm radius and e is the ratio of the convex axial portion radius to R 5. A device according to claim 1 wherein said rotors are cylindrical.

6. A device according to claim 1 wherein successive axial portions of each of said rotors are cylindrical and mutually angularly displaced about the rotor axis in progressive manner.

7. A device according to claim 1 wherein each of said rotors has its arms twisted around and along its axis to spiral form.

8. A device according to claim 7 wherein said spiral is a helix.

9. A device according to claim 8 wherein the pitch of said helix is at least approximately one turn in 21rR /tan 60 of axial length, where R is the maximum radius of the rotor.

10. A device according to claim 1 wherein said rotors are of hollow construction. 

1. A mixing device comprising: a hollow housing defining a cylindrical chamber having symmetrical cross section formed by a plurality of like lobes with circular arcuate peripheries, the circular central axes of adjacent lobes being mutually displaced by a distance less than the sum of the radii thereof, and said housing having port means in at least one end thereof for passage of material to and from said chamber; a plurality of similar rotors individually mounted in respective ones of said lobes for rotation about said central axes, each rotor having like transverse radial cross-sectional form at substantially all points along its axis defined by a plurality of like radially extending arms, each of said arms being defined, in turn, by generally concave radially extending sides and a generally convex radially outer periphery which passes closely adjacent the respective lobe periphery during rotation, and said concave sides of adjacent ones of said arms in each rotor merging in a convex axial portion; and means coupling said rotors for rotation in predetermined intermeshed relation with adjacent rotors rotating at the same rate but in mutually opposite senses; wherein the cross-sectional form of said rotors, and said intermeshing relation, are such that: in each arm the junction of the periphery and sides define a leading tip and a trailing tip relative to the relevant direction of rotation; each leading tip wipes the periphery of its lobe and the trailing tip side of an adjacent rotor arm; and each trailing tip of one arm wipes the leading tip side of an adjacent rotor arm while the periphery of said one arm effectively rolls on and slides over the convex axial portion with which said leading tip side merges.
 2. A device according to claim 1 wherein said arm peripheries and said convex axial portions at least approximate to circular arcuate sectional form.
 3. A device according to claim 2 wherein the ratio between the radii of curvatures of said arm peripheries and convex portions is 1 : 0.3.
 4. A device according to claim 1 wherein each rotor comprises a pair of arms, the 2 of said arms and said convex portions each subtend an angle of substantially 90* at the rotor axis, and each arm side at least approximates in sectional form to a curve defined by the equations: x r cos phi and y r sin phi 0 < theta < arc cos (1 + epsilon /2) R1 is the rotor arm radius and epsilon is the ratio of the convex axial portion radius to R1.
 5. A device according to claim 1 wherein said rotors are cylindrical.
 6. A device according to claim 1 wherein successive axial portions of each of said rotors are cylindrical and mutually angularly displaced about the rotor axis in progressive manner.
 7. A device according to claim 1 wherein each of said rotors has its arms twisted around and along its axis to spiral form.
 8. A device according to claim 7 wherein said spiral is a helix.
 9. A device according to claim 8 wherein the pitch of said helix is at least approximately one turn in 2 pi R1/tan 60* of axial length, where R1 is the maximum radius of the rotor.
 10. A device according to claim 1 wherein said rotors are of hollow construction. 